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Abstract:

A sheet identification device includes plural detecting units, an
estimating unit, and an output unit. The plural detecting units are
provided along a transport path along which a sheet is transported by a
transport unit, and detect passing of the sheet. The estimating unit
estimates a length in a transport direction of the sheet as a detection
target on the basis of individual detection results generated by the
plural detecting units. The output unit outputs, if the length estimated
by the estimating unit on the basis of at least one of the detection
results is not a predetermined length, a signal indicating that a sheet
having a length in the transport direction different from the
predetermined length is being transported.

Claims:

1. A sheet identification device comprising: a plurality of detecting
units that are provided along a transport path along which a sheet is
transported by a transport unit and that detect passing of the sheet; an
estimating unit that estimates a length in a transport direction of the
sheet as a detection target on the basis of individual detection results
generated by the plurality of detecting units; and an output unit that
outputs, if the length estimated by the estimating unit on the basis of
at least one of the detection results is not a predetermined length, a
signal indicating that a sheet having a length in the transport direction
different from the predetermined length is being transported.

2. The sheet identification device according to claim 1, further
comprising: a feed unit that takes a sheet accommodated in an
accommodating unit from the accommodating unit and feeds the sheet to the
transport path; and a controller that controls the feed unit so that, if
a length through which transport is to be assisted by the feed unit is
preset as a length in the transport direction of a sheet to be
transported, a feed operation is performed on the sheet until a
predetermined time period elapses from when the feed unit starts feeding
the sheet to a specific region of the transport path, the predetermined
time period being a time period for feeding the sheet by the preset
length.

3. The sheet identification device according to claim 1, further
comprising: a plurality of feed units that correspond to a plurality of
accommodating units in which sheets to be transported are accommodated,
that take the sheets accommodated in the plurality of accommodating units
from the plurality of accommodating units, and that feed the sheets to
the transport path, the transport unit including a positioning unit that
performs positioning of the sheets fed by the plurality of feed units and
that feeds the sheets to a specific region of the transport path; and a
supplying unit that corresponds to at least one of the plurality of
accommodating units, that receives a sheet fed by a corresponding one of
the plurality of feed units, and that feeds the received sheet to the
positioning unit, thereby supplying the sheet to the positioning unit,
wherein the estimating unit estimates, if the supplying unit corresponds
to one of the plurality of accommodating units from which the sheet has
been taken by one of the plurality of feed units, a length in the
transport direction of the sheet as a detection target on the basis of a
detection result generated by one of the plurality of detecting units,
and estimates, if the supplying unit does not correspond to one of the
plurality of accommodating units from which the sheet has been taken by
one of the plurality of feed units, a length in the transport direction
of the sheet as a detection target on the basis of detection results
generated by the plurality of detecting units.

4. The sheet identification device according to claim 3, wherein the
estimating unit estimates, if the supplying unit corresponds to one of
the plurality of accommodating units from which the sheet has been taken
by one of the plurality of feed units, a length in the transport
direction of the sheet as a detection target on the basis of a detection
result generated by a detecting unit on an upstream side among the
plurality of detecting units, and estimates, if the supplying unit does
not correspond to one of the plurality of accommodating units from which
the sheet has been taken by one of the plurality of feed units, a length
in the transport direction of the sheet as a detection target on the
basis of detection results generated by the plurality of detecting units.

5. The sheet identification device according to claim 3, wherein the
estimating unit estimates, if the supplying unit corresponds to one of
the plurality of accommodating units from which the sheet has been taken
by one of the plurality of feed units, a length in the transport
direction of the sheet as a detection target on the basis of a detection
result generated by a detecting unit on a downstream side among the
plurality of detecting units, and estimates, if the supplying unit does
not correspond to one of the plurality of accommodating units from which
the sheet has been taken by one of the plurality of feed units, a length
in the transport direction of the sheet as a detection target on the
basis of detection results generated by the plurality of detecting units.

6. An image forming apparatus comprising: the sheet identification device
according to claim 1; and an image forming unit that forms an image
corresponding to image information on a sheet transported to the
transport path by the transport unit.

7. A computer readable medium storing a program causing a computer to
execute a process for controlling an image forming apparatus including a
plurality of detecting units that are provided along a transport path
along which a sheet is transported by a transport unit and that detect
passing of the sheet, the process comprising: estimating a length in a
transport direction of the sheet as a detection target on the basis of
individual detection results generated by the plurality of detecting
units; and outputting, if the length estimated on the basis of at least
one of the detection results is not a predetermined length, a signal
indicating that a sheet having a length in the transport direction
different from the predetermined length is being transported.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on and claims priority under 35 USC 119
from Japanese Patent Application No. 2011-126291 filed Jun. 6, 2011.

BACKGROUND

(i) Technical Field

[0002] The present invention relates to a sheet identification device, an
image forming apparatus, and a computer readable medium.

SUMMARY

[0003] According to an aspect of the invention, there is provided a sheet
identification device including plural detecting units, an estimating
unit, and an output unit. The plural detecting units are provided along a
transport path along which a sheet is transported by a transport unit,
and detect passing of the sheet. The estimating unit estimates a length
in a transport direction of the sheet as a detection target on the basis
of individual detection results generated by the plural detecting units.
The output unit outputs, if the length estimated by the estimating unit
on the basis of at least one of the detection results is not a
predetermined length, a signal indicating that a sheet having a length in
the transport direction different from the predetermined length is being
transported.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Exemplary embodiments of the present invention will be described in
detail based on the following figures, wherein:

[0005]FIG. 1 is a side cross-sectional view illustrating an example of a
configuration of an image forming apparatus according to an exemplary
embodiment of the present invention;

[0006]FIG. 2 is a schematic view illustrating an example of disposition
of a first path sensor and a second path sensor according to the
exemplary embodiment;

[0007]FIG. 3 is a diagram illustrating an example of a configuration of
an image forming part according to the exemplary embodiment;

[0008]FIG. 4 is a block diagram illustrating a configuration of an
electric system of the image forming apparatus according to the exemplary
embodiment;

[0009] FIGS. 5A and 5B illustrate a flowchart of a process of a sheet
length identification process program according to a first exemplary
embodiment of the present invention;

[0010]FIG. 6 is a flowchart illustrating a process of a transport start
process program according to the first exemplary embodiment; and

[0011] FIGS. 7A and 7B illustrate a flowchart of a process of a sheet
length identification process program according to an accommodating unit
according to a second exemplary embodiment of the present invention.

DETAILED DESCRIPTION

[0012] Hereinafter, exemplary embodiments of the present invention will be
described in detail with reference to the drawings.

First Exemplary Embodiment

[0013]FIG. 1 is a side cross-sectional view of an image forming apparatus
10 according to a first exemplary embodiment of the present invention.
The image forming apparatus 10 includes a sheet accommodating unit 12, an
image forming unit 14, an output unit 16, a document reader 18, and a
controller 20, which are disposed from the lower side toward the upper
side in the vertical direction in a front view of FIG. 1 (Y direction
shown in FIG. 1). The sheet accommodating unit 12 accommodates sheets of
recording paper P. The image forming unit 14 is provided above the sheet
accommodating unit 12 and forms images on sheets of recording paper P fed
from the sheet accommodating unit 12. The output unit 16 is provided
above and to the left of the image forming unit 14 by being integrated
with the image forming unit 14, and outputs sheets of recording paper P
on which images have been formed. The document reader 18 is provided
above the output unit 16 and reads a document G. The controller 20 is
provided in the image forming unit 14 and controls the operations of the
individual units of the image forming apparatus 10. In the description
given below, the vertical direction in a front view of FIG. 1 is referred
to as a Y direction, and the horizontal direction is referred to as an X
direction. Also, "right" and "left" mean right and left in a front view
of the image forming apparatus 10.

[0014] The sheet accommodating unit 12 includes a first accommodating unit
22, a second accommodating unit 24, a third accommodating unit 26, and a
fourth accommodating unit 28 arranged along the Y direction, which
accommodate sheets of recording paper P of different sizes. Each of the
first accommodating unit 22, the second accommodating unit 24, the third
accommodating unit 26, and the fourth accommodating unit 28 (hereinafter
referred to as "accommodating unit(s)" when it is not necessary to
distinguish between them) is provided with a feed roller 32 for feeding a
sheet of recording paper P accommodated therein to a transport path 30.
The transport path 30 is defined by a plate-like chute 31 that guides a
sheet of recording paper P fed by the feed roller 32 from the sheet
accommodating unit 12 to the image forming unit 14 and the output unit
16. The chute 31 includes a main chute 31A and branch chutes 31B1 to
31B4. The main chute 31A guides a sheet of recording paper P from
the sheet accommodating unit 12 to the image forming unit 14, and defines
a backbone portion of the transport path 30. The branch chutes 31B1
to 31B4 branch off from the main chute 31A, receive sheets of
recording paper P fed by the respective feed rollers 32 from the first to
fourth accommodating units 22, 24, 26, and 28, respectively, and guide
the sheets of recording paper P to the main chute 31A. The branch chute
31B1 corresponds to the first accommodating unit 22. The branch
chute 31B2 corresponds to the second accommodating unit 24. The
branch chute 31B3 corresponds to the third accommodating unit 26.
The branch chute 31B4 corresponds to the fourth accommodating unit
28. Thus, the inlets of sheets of recording paper P in the respective
branch chutes 31B1 to 31B4 function as the inlet of the
transport path 30.

[0015] The main chute 31A is provided with pairs of transport rollers 36,
which correspond to the respective outlets of the branch chutes 31B1
to 31B3. Each pair of transport rollers 36 is constituted by a pair
of rollers that sandwich a sheet of recording paper P fed by the
corresponding feed roller 32 and feed the sheet to the downstream side in
a transport direction. The pair of transport rollers 36 having such a
configuration temporarily stops a sheet of recording paper P fed by the
feed roller 32 at a nip portion and feeds the sheet to the downstream
side in the transport direction at a predetermined timing in accordance
with control performed by the controller 20.

[0016] On the other hand, no pair of transport rollers 36 is provided for
the branch chute 31B4. Thus, a sheet of recording paper P fed by the
feed roller 32 to the branch chute 31B4 is guided to the main chute
31A by the driving force of the feed roller 32, and is fed to the
downstream side in the transport direction.

[0017] In the main chute 31A, a pair of positioning rollers 38 is provided
on the downstream side of the pairs of transport rollers 36 in the
transport direction of a sheet of recording paper P. The pair of
positioning rollers 38 temporarily stops a sheet of recording paper P and
feeds the sheet to a downstream region (second transfer unit 37) at a
predetermined timing. The pair of positioning rollers 38 is disposed such
that the peripheral surfaces of the rollers face each other and are in
contact with each other. The pair of positioning rollers 38 sandwich a
sheet of recording paper P and are rotated by receiving a driving force
from a motor 14A (see FIG. 4), thereby feeding the sheet to the
downstream side of the transport path 30. The pair of positioning rollers
38 are configured so as not to be rotated by the weight of the pair of
positioning rollers 38 and the weight of a gear that connects the pair of
positioning rollers 38 and the drive shaft of the motor 14A, even if the
leading end of a sheet of recording paper P fed by the feed roller 32
comes into contact with the nip portion and is pushed.

[0018] The image forming unit 14 includes a casing 16A, which serves as a
body of the apparatus. The casing 16A is configured so that the upper
left portion of the image forming unit 14 protrudes toward the upper side
with respect to the upper center portion and the upper right portion in a
front view of the image forming apparatus 10. The left end portion of the
document reader 18 is connected to the upper end of the output unit 16.
Accordingly, the image forming apparatus 10 has an output region 19
surrounded by the upper surface of the image forming unit 14, the lower
surface of the document reader 18, and the right surface of the output
unit 16. Sheets of recording paper P from the output unit 16 are output
and stacked in the output region 19.

[0019] An auxiliary transport path 40 is provided on the opposite side of
the transport path 30 for the image forming unit 14 to the fourth
accommodating unit 28. A sheet of recording paper P fed from a foldable
manual sheet feeder 39, which is provided on the left surface of the
image forming apparatus 10 in a front view of the image forming apparatus
10, is transported along the auxiliary transport path 40 to the transport
path 30. The auxiliary transport path 40 is provided with a feed roller
42, which feeds a sheet of recording paper P fed from the manual sheet
feeder 39 to the auxiliary transport path 40, and plural pairs of
transport rollers 44, which are provided on the downstream side of the
feed roller 42 and which transport sheets of recording paper P one by
one. The downstream-side end of the auxiliary transport path 40 is
connected to the transport path 30.

[0020] Also, a fixing device 100 is provided on the downstream side of the
second transfer unit 37 along the transport path 30 in the image forming
unit 14. The fixing device 100 includes a fixing roller 102 for heating a
developer on a sheet of recording paper P, and a pressure roller 104 for
pressing the sheet against the fixing roller 102. The fixing device 100
is configured so that, after a sheet of recording paper P has passed
through a nip portion N, where the fixing roller 102 is in contact with
the pressure roller 104, a developer is fused and fixed, whereby a
developer image is fixed onto the sheet. In the image forming apparatus
10 according to the first exemplary embodiment, a two-component developer
including toner and carrier is adopted as a developer. Hereinafter, the
developer is referred to as "toner".

[0021] The image forming unit 14 includes a first path sensor 91A and a
second path sensor 91B that detect passing of a sheet of recording paper
P in a predetermined region of the transport path 30. In the image
forming apparatus 10, various types of control (for example, control of
the timing to perform transfer, timing to start/stop rotation of each
roller, and timing to perform a fixing process) is performed on the basis
of detection results generated by the first path sensor 91A and the
second path sensor 91B.

[0022]FIG. 2 schematically illustrates an example in which the first path
sensor 91A and the second path sensor 91B according to the first
exemplary embodiment are disposed along the transport path 30. As
illustrated in FIG. 2, the pair of transport rollers 36, the pair of
positioning rollers 38, the second transfer unit 37, and the fixing
device 100 are disposed along the transport path 30 in this order from
the upstream side toward the downstream side in the transport direction.
The first path sensor 91A is disposed on the upstream side of the pair of
positioning rollers 38 and on the downstream side of the pair of
transport rollers 36 in the transport direction along the transport path
30. On the other hand, the second path sensor 91B is disposed on the
downstream side of the fixing device 100 along the transport path 30. The
first path sensor 91A and the second path sensor 91B are normally in an
ON-state, and shift to an OFF-state while a sheet of recording paper P is
passing through a detection target region. In other words, the first path
sensor 91A and the second path sensor 91B are in an ON-state when no
sheet of recording paper P exists in the detection target region
(non-passing period of a sheet of recording paper P), and are in an
OFF-state when a sheet of recording paper P exists in the detection
target region (passing period of a sheet of recording paper P). A
reflective photo interrupter is used for both the first path sensor 91A
and second path sensor 91B according to the first exemplary embodiment.
Alternatively, a transparent photo interrupter may be used therefor, and
any types of sensor may be used as long as the sensor detects passing of
a sheet of recording paper P (for example, a sensor that detects the
leading end and the trailing end of a sheet of recording paper P).

[0023] As illustrated in FIGS. 1 and 3, an image forming part 60 is
provided at the center of the image forming unit 14. The image forming
part 60 serves as a forming part for forming a toner image on a sheet of
recording paper P by mixing toners of respective colors of black (K),
yellow (Y), magenta (M), and cyan (C). The image forming part 60 includes
photoconductor drums 62K, 62Y, 62M, and 62C for holding electrostatic
latent images, which correspond to toners of respective colors of K, Y,
M, and C. In the description given below, when it is necessary to
distinguish K, Y, M, and C from one another, corresponding one of K, Y,
M, and C will be attached after reference numeral. When it is not
necessary to distinguish them, K, Y, M, and C will be omitted.

[0024] The photoconductor drums 62K, 62Y, 62M, and 62C are arranged in
this order toward the upper right side in FIG. 3, are rotated in the
direction indicated by an arrow b (counterclockwise in the figure), and
hold an electrostatic latent image formed through irradiation on their
peripheral surfaces. Also, a charging roller 66, a light-emitting diode
(LED) head 68, a developing device 72, an intermediate transfer belt 64,
and a cleaning roller 76 are provided in this order along the arrow-b
direction around each of the photoconductor drums 62K, 62Y, 62M, and 62C.

[0025] The charging roller 66 has a configuration in which plural layers
including an electro-conductive elastic layer, an intermediate layer, and
a surface resin layer (not illustrated) are formed around a shaft made of
stainless steel, for example. The shaft of the charging roller 66 is
rotatable so that the charging roller 66 is rotated in conjunction with
the photoconductor drum 62, with the peripheral surface of the charging
roller 66 being in contact with the surface layer of the photoconductor
drum 62. The charging roller 66 causes the peripheral surface of the
photoconductor drum 62 to be charged through discharge that occurs when a
voltage is applied from a voltage applying unit (not illustrated).

[0026] The LED head 68 causes the peripheral surface of the photoconductor
drum 62 charged by the charging roller 66 to be irradiated with (exposed
to) light corresponding to the color of toner, thereby forming an
electrostatic latent image. Alternatively, a method for scanning laser
light for four colors of K, Y, M, and C in common using a polygon mirror
may be used as a method for exposing the photoconductor drums 62.

[0027] The developing device 72 includes a developing roller 71 that
supplies toner to an electrostatic latent image formed on the
photoconductor drum 62 and forms a toner image, and transport members 73A
and 73B that transport toner to the developing roller 71 in a circulating
manner.

[0028] The intermediate transfer belt 64 is an endless belt, is wound
around a belt transport roller 82 provided in the second transfer unit
37, a belt transport roller 84 provided below and to the right of the
belt transport roller 82, and a driving roller 86 that is provided above
and to the right of the belt transport roller 82 and is driven by a motor
(not illustrated), and is supported so as to be rotatably moved in the
direction indicated by an arrow a (clockwise in the figure). The outer
surface of the intermediate transfer belt 64 serves as a transfer surface
onto which a toner image is to be transferred. The peripheral surfaces of
the photoconductor drums 62K, 62Y, 62M, and 62C are in contact with a
transfer surface of the intermediate transfer belt 64, the transfer
surface extending from the driving roller 86 to the belt transport roller
84.

[0029] First transfer rollers 74 (74K, 74Y, 74M, and 74C) are provided on
the opposite side to the photoconductor drums 62K, 62Y, 62M, and 62C,
with the intermediate transfer belt 64 interposed therebetween. The first
transfer rollers 74 are in contact with the inner surface of the
intermediate transfer belt 64. When a voltage is applied to the first
transfer rollers 74 from a voltage applying unit (not illustrated), the
first transfer rollers 74 perform first transfer, thereby transferring
the toner images on the photoconductor drums 62 onto the transfer surface
of the intermediate transfer belt 64 using a potential difference between
the first transfer rollers 74 and the photoconductor drums 62 that are
grounded. Accordingly, the individual toner images are transferred onto
the intermediate transfer belt 64 in a superimposed manner while the
intermediate transfer belt 64 makes a round.

[0030] Also, a toner density detection sensor 88 is provided on the
opposite side to the belt transport roller 84, with the intermediate
transfer belt 64 interposed therebetween. The tonner density detection
sensor 88 has a function of detecting the density of a toner image
transferred onto the transfer surface of the intermediate transfer belt
64. Furthermore, a cleaning member 92 is provided on the opposite side to
the driving roller 86, with the intermediate transfer belt 64 interposed
therebetween. The cleaning member 92 removes residual toner that is left
on the transfer surface of the intermediate transfer belt 64 after second
transfer.

[0031] The second transfer unit 37 includes the belt transport roller 82
on which the intermediate transfer belt 64 is wound, and a second
transfer roller 89. The second transfer roller 89 serves as a transport
unit and is provided on the opposite side to the belt transport roller
82, with the intermediate transfer belt 64 interposed therebetween. The
second transfer roller 89 is connected to a voltage applying unit 90 so
that a voltage is applied thereto from the voltage applying unit 90, and
is configured so that the toner image on the intermediate transfer belt
64 is second transferred onto a sheet of recording paper P due to a
potential difference between the belt transport roller 82 and the second
transfer roller 89.

[0032] As illustrated in FIG. 1, toner cartridges 77K, 77Y, 77M, and 77C
containing toners of K, Y, M, and C, respectively, are provided in an
exchangeable manner to the right of the cleaning member 92 in the image
forming unit 14. Also, a duplex-printing transport path 94 is provided to
the left of the transport path 30 in the image forming unit 14. In the
duplex-printing transport path 94, a sheet of recording paper P is
transported and reversed so as to form images on both sides of the sheet.

[0033] One end of the duplex-printing transport path 94 is connected
between a pair of transport rollers 95 and a pair of transport rollers
96. The pair of transport rollers 95 is provided on the downstream side
of the fixing device 100 in the transport direction of a sheet of
recording paper P along the transport path 30. The pair of transport
rollers 96 is provided on the downstream side of the pair of transport
rollers 95, and the rotation direction thereof may be switched. The other
end of the duplex-printing transport path 94 is connected to the upstream
side of the pair of positioning rollers 38. Also, the duplex-printing
transport path 94 is provided with plural transport rollers 97 that
transport a sheet of recording paper P fed from the pair of transport
rollers 96 toward the pair of positioning rollers 38. Accordingly, when
images are to be formed on both sides, a sheet of recording paper P to
which a toner image has been fixed onto the front side by the fixing
device 100 is fed to the duplex-printing transport path 94 due to a
reverse rotation of the pair of transport rollers 96 and a path switching
member (not illustrated), and is fed again to the pair of positioning
rollers 38, so that the front and back sides of the sheet of recording
paper P are reversed.

[0034] Lower output rollers 54 are provided on the downstream side of the
pair of transport rollers 95 in the output unit 16. The lower output
rollers 54 are disposed along a transport path 33, which branches off
from the transport path 30 to the output region 19 side, and output a
sheet of recording paper P onto a lower table 52 provided in an upper
section of the image forming unit 14. A lower detecting unit 55 is
provided at a position next to the lower output rollers 54. The lower
detecting unit 55 detects the height of sheets of recording paper P
stacked on the lower table 52. Also, upper output rollers 57 are provided
on the downstream side of the pair of transport rollers 95 along the
transport path 30 in the output unit 16. The upper output rollers 57
output sheets of recording paper P onto an upper table 56, which is
provided above the lower table 52. An upper detecting unit 58 is provided
at a position next to the upper output rollers 57. The upper detecting
unit 58 detects the height of sheets of recording paper P stacked on the
upper table 56.

[0035] The document reader 18 includes a document transport device 45 that
automatically transports sheets of a document G one by one, a platen
glass 47 which is disposed below the document transport device 45 and on
which one sheet of the document G is placed, and a document reading
device 49 that reads the document G transported by the document transport
device 45 or the document G placed on the platen glass 47. The document
transport device 45 includes an automatic transport path 48 along which
plural pairs of transport rollers 46 are disposed. Part of the automatic
transport path 48 is disposed so that a sheet of recording paper P passes
over the platen glass 47. The document reading device 49 is configured to
read a document G that is transported by the document transport device 45
in the state of being stationary at the left end of the platen glass 47,
or to read a document G placed on the platen glass 47 while moving in the
X direction.

[0036]FIG. 4 is a block diagram illustrating a configuration of an
electric system of the image forming apparatus 10 according to this
exemplary embodiment. As illustrated in FIG. 4, the image forming
apparatus 10 includes a central processing unit (CPU) 150 serving as an
estimating unit and a controller, a read only memory (ROM) 152, a random
access memory (RAM) 154, a second memory 156, a user interface (UI) panel
158, and an external interface 160.

[0037] The ROM 152 functions as a memory that stores in advance a control
program for controlling the operation of the image forming apparatus 10,
a sheet length identification process program described below, and
various parameters. The RAM 154 is used as a working area or the like for
executing various programs. The second memory 156 stores various pieces
of information that are to be held even after the power switch of the
apparatus is turned off. For example, a hard disk device or a flash
memory is applied thereto. The UI panel 158 is constituted by a touch
panel display or the like in which a transparent touch panel is
superimposed on a display. Various pieces of information are displayed on
the display screen of the display. Also, the UI panel 158 receives
various pieces of information and instructions when the touch panel is
touched by a user. The external interface 160 is connected to the
external apparatus 162, such as a personal computer, receives various
pieces of information, such as image information representing an image
that is to be formed on recording paper P, from the external apparatus
162, and transmits various pieces of information, such as information
representing the status of the image forming apparatus 10, to the
external apparatus 162.

[0038] The image forming unit 14, the CPU 150, the ROM 152, the RAM 154,
the second memory 156, the UI panel 158, and the external interface 160
are connected to one another via a system bus BUS. Thus, the CPU 150
accesses the ROM 152, the RAM 154, and the second memory 156, displays
various pieces of information on the UI panel 158, recognizes operation
instructions provided by a user to the UI panel 158, receives various
pieces of information from the external apparatus 162 via the external
interface 160, transmits various pieces of information to the external
apparatus 162 via the external interface 160, recognizes the operation
status of the image forming unit 14, and controls the operation of the
image forming unit 14.

[0039] The image forming unit 14 serving as an image forming unit
includes, for example, the motor 14A that generates a driving force for
rotating various rollers for transporting a sheet of recording paper P,
such as the feed rollers 32 serving as a feed unit, the pairs of
transport rollers 36 serving as a supplying unit, the pairs of transport
rollers 44, 54, 57, 95, and 96 serving as a transport unit, the pair of
positioning rollers 38 serving as a transport unit, the fixing roller 102
serving as a transport unit, and the pressure roller 104 serving as a
transport unit. Also, the image forming unit 14 includes a motor
controller 14B that is connected to the motor 14A and controls driving of
the motor 14A, the first path sensor 91A serving as a detecting unit, and
the second path sensor 91B serving as a detecting unit. The motor
controller 14B, the first path sensor 91A, and the second path sensor 91B
are connected to one another via the system bus BUS. Thus, the CPU 150
controls the motor 14A via the motor controller 14B, recognizes the
operation status of the motor 14A via the motor controller 14B, and
receives signals output from the first path sensor 91A and the second
path sensor 91B (ON signal representing an ON state and OFF signal
representing an OFF state).

[0040] The image forming apparatus 10 according to the first exemplary
embodiment has an assist function of assisting transport of a sheet of
recording paper P by using the feed roller 32 in a case where the length
in the transport direction of the sheet to be transported (hereinafter
referred to as "sheet length") is equal to or larger than a predetermined
length (for example, the length in the longitudinal direction of the A4
size).

[0041] The assist function is a function of assisting transport of a sheet
of recording paper P while the sheet is being transported along the
transport path 30 in order to suppress the occurrence of wrinkling or
slipping of the sheet during transport. It is necessary that the
following three conditions (i) to (iii) are satisfied in order to operate
the assist function (hereinafter the three conditions are referred to as
"assist function operation conditions").

[0042] (i) A sheet length equal to or larger than a predetermined length
is set.

[0043] (ii) The leading end of a sheet of recording paper P is in contact
with the pair of positioning rollers 38 the rotation of which is stopped.

[0044] (iii) The rotation of the feed roller 32 is stopped.

[0045] When the assist function operation conditions are satisfied, the
assist function simultaneously starts the rotation of the pair of
positioning rollers 38 and the feed roller 32 that are temporarily
stopped, and continues the rotation of the feed roller 32 until a
predetermined stop condition is satisfied (for example, a predetermined
time period has elapsed as a time period for assisting transport of a
sheet of recording paper P having a sheet length set by a user after the
rotation of the pair of positioning rollers 38 and the feed roller 32
that are temporarily stopped has simultaneously started (after feeding to
the downstream side of the pair of positioning rollers 38 has started)).
The predetermined stop condition is a predetermined condition for
determining that the trailing end of a sheet of recording paper P has
passed the feed roller 32, for example, a condition in which a
predetermined time period (for example, 0.8 seconds) has elapsed from
restart of transport of the sheet.

[0046] If the condition "a sheet length smaller than a predetermined
length is set" is satisfied instead of condition (i), the assist function
is not operated, and a normal transport operation is performed. Here, the
"normal transport operation" means transporting a sheet of recording
paper P by rotating the pair of positioning rollers 38 and the individual
rollers disposed along the transport path 30, not by rotating the feed
roller 32. In the image forming apparatus 10 according to the first
exemplary embodiment, "setting of a sheet length" means determining a
sheet length on the basis of the position of a side fence (not
illustrated) provided in the accommodating unit that accommodates sheets
of recording paper P as a target to be transported (target on which an
image is to be formed) and that is specified via the UI panel 158, and
storing the determined sheet length in a predetermined storage region.
Here, the "side fence" is a fence for specifying the length of sheets of
recording paper P accommodated in the accommodating unit. The position of
the side fence in the accommodating unit may be detected by using a
sensor, such as a photo interrupter or a micro switch.

[0047] In the image forming apparatus 10 according to the first exemplary
embodiment, a side fence is provided in each accommodating unit, but the
side fence may not be provided in each accommodating unit. In that case,
"setting of a sheet length" means providing accommodating units
corresponding to individual sheet lengths of recording paper P,
specifying any one of the accommodating units via the UI panel 158, and
storing the sheet length corresponding to the specified accommodating
unit in a predetermined storage region.

[0048] Next, the operation of the image forming apparatus 10 according to
this exemplary embodiment will be described.

[0049] First, an image formation process performed by the image forming
apparatus 10 will be described.

[0050] After the image forming apparatus 10 has been activated, image data
of individual colors of K, Y, M, and C is output from an image processing
apparatus (not illustrated) or the outside to the corresponding LED heads
68 (see FIG. 3). Subsequently, light is emitted from the LED heads 68 in
accordance with the image data, the peripheral surfaces of the
photoconductor drums 62 charged by the charging rollers 66 are exposed to
the light, and electrostatic latent images corresponding to the image
data of the respective colors are formed on the surfaces of the
respective photoconductor drums 62. The electrostatic latent images
formed on the surfaces of the respective photoconductor drums 62 are
developed into toner images by the respective developing devices 72.
Then, the toner images on the surfaces of the respective photoconductor
drums 62 are sequentially transferred onto the intermediate transfer belt
64 by the first transfer rollers 74.

[0051] On the other hand, a sheet of recording paper P that is fed from
the sheet accommodating unit 12 and is transported along the transport
path 30 is transported by the pair of positioning rollers 38 to the
second transfer unit 37 in synchronization with the transfer of the
individual toner images onto the intermediate transfer belt 64. The toner
images that have been transferred onto the intermediate transfer belt 64
are second transferred by the second transfer roller 89 onto the sheet of
recording paper P transported to the second transfer unit 37.

[0052] Subsequently, the sheet of recording paper P onto which the toner
images have been transferred is transported to the fixing device 100. In
the fixing device 100, the toner images are heated and pressed by the
fixing roller 102 and the pressure roller 104, thereby being fixed onto
the sheet of recording paper P. Furthermore, the sheet of recording paper
P onto which the toner images have been fixed is output from the output
unit 16 to the lower table 52 or the upper table 56. In the case of
forming images on both sides of the sheet of recording paper P, an image
is fixed onto the front surface of the sheet by the fixing device 100,
and the trailing end of the sheet is fed from the pair of transport
rollers 96 to the duplex-printing transport path 94 and to the pair of
positioning rollers 38 (transport path 30), so that the leading end and
the trailing end of the sheet are changed. Then, an image is formed on
and fixed onto the rear surface of the sheet.

[0053] In the image forming apparatus 10 according to the first exemplary
embodiment, the length of a sheet of recording paper P to be transported
is set by a user, as described above. Also, in the image forming
apparatus 10, an image is formed on the sheet of recording paper P
accommodated in the accommodating unit corresponding to the set sheet
length. However, if sheets of recording paper P having a sheet length not
intended by the user are accommodated in the accommodating unit, an image
formation process is performed on a sheet of recording paper P having a
sheet length different from the sheet length set by the user. For
example, if the length of a sheet of recording paper P that is being
transported is smaller than the sheet length set by the user, toner may
be applied to a region where no sheet exists, so that the inside of the
image forming unit 14 may become dirty unnecessarily. In order to avoid
the occurrence of such a situation, passing of a sheet of recording paper
P may be detected at the inlet of the transport path 30 to measure the
sheet length, and it may be determined whether or not the sheet length is
a sheet length intended by the user. Accordingly, the occurrence of a
situation in which an image formation process is performed on a sheet of
recoding paper P having an unintended sheet length is prevented. However,
for example, if sheets of recording paper P are not fed one by one by the
feed roller 32, a sheet of recording paper P may be fed to the transport
path 30 in the state of being partially overlapped on the preceding
sheet. The cause of such overlap may be, for example, burrs caused by
punching a hole in the recording paper P using a puncher, static
electricity on the recording paper P, or an adhesive (for example, glue)
that is adhered to the recording paper P in an unintended manner. If a
sheet of recording paper P is transported in the state of being partially
overlapped on the subsequent sheet due to such a cause, a wrong
determination may be made in which a sheet of recording paper P having a
length intended by the user (sheet length set by the user) is being
transported, although the sheet length is actually different from that
intended by the user. Also, another wrong determination may be made in
which a sheet of recording paper P having a length intended by the user
is not being transported, although the sheet length is actually the sheet
length intended by the user.

[0054] Accordingly, in the image forming apparatus 10 according to this
exemplary embodiment, a sheet length identification process is performed
in which it is identified with high accuracy whether or not the length of
a sheet of recording paper P that is currently being transported is a
sheet length set by a user.

[0055] Hereinafter, the operation of the image forming apparatus 10 during
the execution of the sheet length identification process will be
described with reference to FIGS. 5A and 5B. FIGS. 5A and 5B illustrate a
flowchart of an example of a process of a sheet length identification
process program executed by the CPU 150 when the rotation of the feed
roller 32 corresponding to the fourth accommodating unit 28 is started in
order to take a sheet of recording paper P accommodated in the fourth
accommodating unit 28 and to feed the sheet to the branch chute
31B4. The sheet length identification process program is stored in
advance in a predetermined storage region of the ROM 152. Here, in order
to avoid complexity, a description will be given of a case where the
sheet length identification process program is executed in a state where
the rotation of the pair of positioning rollers 38 and the rollers
disposed along the transport path 30 is stopped.

[0056] Referring to FIGS. 5A and 5B, in step 200, it is determined whether
or not the first path sensor 91A has shifted from an OFF-state to an
ON-state. If a positive determination is made, the process proceeds to
step 202. If a negative determination is made, the process proceeds to
step 238. In step 238, it is determined whether or not a predetermined
condition is satisfied. The predetermined condition is a condition for
determining that a sheet of recording paper P having a predetermined
sheet length is not being transported (for example, one second has passed
from the start of the execution of step 200). If a negative determination
is made, the process returns to step 200. If a positive determination is
made, the process proceeds to step 232.

[0057] In step 202, the process waits until the leading end of a sheet of
recording paper P fed to the chute 31 by the feed roller 32 comes into
contact with the pair of positioning rollers 38 and the sheet forms
bending of a predetermined degree (bending for correcting skew), that is,
until 0.5 seconds have elapsed from the start of the rotation of the feed
roller 32. Then, the process proceeds to step 203, where the rotation of
the feed roller 32 is stopped, and the process proceeds to step 204.

[0058] In step 204, a transport start process is performed. Now, the
operation of the image forming apparatus 10 during the execution of the
transport start process will be described with reference to FIG. 6. FIG.
6 is a flowchart illustrating an example of the process of a transport
start process program executed by the CPU 150. The transport start
process program is stored in advance in a predetermined storage region of
the ROM 152.

[0059] In step 204A in FIG. 6, it is determined whether or not the assist
function operation conditions are satisfied. If a positive determination
is made, the process proceeds to step 204B. If a negative determination
is made, the process proceeds to step 204C. In step 204B, the assist
function is operated, and the rotation of the rollers disposed along the
transport path 30 is started. Then, the transport start process program
is ended, and the process proceeds to step 206 of the sheet length
identification process program. On the other hand, in step 204C, the
rotation of the individual rollers disposed along the transport path 30
is started, thereby starting transport of a sheet of recording paper P.
Then, the transport start process program is ended, and the process
proceeds to step 206 of the sheet length identification process program.

[0060] In step 206 in FIG. 5A, measuring the time is started. Then, the
process proceeds to step 208, where it is determined whether or not the
first path sensor 91A has shifted from the ON-state to the OFF-state. If
a positive determination is made, the process proceeds to step 210. If a
negative determination is made, the process proceeds to step 240. In step
240, it is determined whether or not a predetermined condition is
satisfied. The predetermined condition is a condition for determining
that a sheet of recording paper P having a predetermined sheet length is
not being transported (for example, one second has passed from the start
of measurement of the time in step 206). If a negative determination is
made, the process returns to step 208. If a positive determination is
made, the process proceeds to step 232.

[0061] In step 210, the measurement of the time started in step 206 is
stopped. In step 212, the length of the sheet of recording paper P that
is currently being transported is estimated on the basis of the time
measured in steps 206 and 210, and then the process proceeds to step 214.
In step 212, the sheet length is estimated by using the following
equation (1). Alternatively, the sheet length may be estimated by
obtaining the sheet length by using a look-up table to which the time
measured in steps 206 and 210 is input and from which the sheet length is
output.

Sheet length (mm)=V×(T1-α)+A (1)

[0062] Here, T1 (sec) represents the time measured (the elapsed time from
step 206 to step 210), V (mm/sec) represents the transport speed of a
sheet of recording paper P, A (mm) represents the distance from a
position of the pair of positioning rollers 38 where the leading end of a
sheet of recording paper P comes into contact (nip portion) to a
detection position of the first path sensor 91A, and a (sec) represents a
waiting time until transport restarts (for example, the sum of a time
period in which the rotation of the feed roller 32 temporarily stops, and
a time period from when an instruction to operate the pair of positioning
rollers 38 is provided to when the pair of positioning rollers 38 are
actually operated).

[0063] In step 214, it is determined whether or not the sheet length
estimated in step 212 is out of an assumed range. If a positive
determination is made, the process proceeds to step 216, where a flag
indicating that the length of the sheet of recording paper P that is
currently being transported is out of the assumed range is set (to an
ON-state), and the process proceeds to step 218. If a negative
determination is made in step 214, step 216 is skipped and the process
proceeds to step 218. Here, the "length out of the assumed range" is
determined in view of an error of ±1 mm if the size of a sheet of
recording paper P specified as a target to be transported is A3SEF, for
example.

[0064] In step 218, it is determined whether or not the second path sensor
91B has shifted from the OFF-state to the ON-state. If a positive
determination is made, the process proceeds to step 220. If a negative
determination is made, the process proceeds to step 242. In step 242, it
is determined whether or not a predetermined condition is satisfied. The
predetermined condition is a condition for determining that a sheet of
recording paper P having a predetermined sheet length is not being
transported (for example, 0.5 seconds have passed since the second path
sensor 91B shifted from the ON-state to the OFF-state). If a negative
determination is made, the process returns to step 218. If a positive
determination is made, the process proceeds to step 232.

[0065] In step 220, measurement of time is started. Then, the process
proceeds to step 222, where it is determined whether or not the second
path sensor 91B has shifted from the ON-state to the OFF-state. If a
positive determination is made, the process proceeds to step 224. If a
negative determination is made, the process proceeds to step 244. In step
244, it is determined whether or not a predetermined condition is
satisfied. The predetermined condition is a condition for determining
that a sheet of recording paper P having a predetermined sheet length is
not being transported (for example, one second has passed from the start
of measurement of time in step 220). If a negative determination is made,
the process returns to step 222. If a positive determination is made, the
process proceeds to step 232.

[0066] In step 224, the measurement of time started in step 220 is
stopped. In step 226, the length of the sheet of recording paper P that
is currently being transported is estimated on the basis of the measured
time, and the process proceeds to step 228.

[0067] In step 228, it is determined whether or not the sheet length
estimated in step 226 is out of the assumed range. If a positive
determination is made, the process proceeds to step 230, where a flag
indicating that the length of the sheet of recording paper P that is
currently being transported is out of the assumed range is set (to an
ON-state), and then the process proceeds to step 232.

[0068] In step 232, an alarm signal indicating that the length of the
sheet of recording paper P that is currently being transported is out of
the assumed range is output, and then the process proceeds to step 234.
The alarm signal is output to the UI panel 158, for example. When
receiving the alarm signal, the UI panel 158 displays a message
indicating that the length of the sheet of recording paper P that is
currently being transported is out of the assumed range. An example of
the message may be "The set sheets have a size different from the
specified size.", "The specified sheet is not being transported.", or
"Please set the sheet size again." Instead of displaying a visible
message on the UI panel 158, an audible message may be output from a
speaker, or a message may be printed on paper so that the message may be
permanently visible. Alternatively, some of display of a visible message,
output of an audible message, and permanent display of a message may be
used together. Alternatively, an alarm signal may be transmitted to the
external apparatus 162 via the external interface 160.

[0069] If a negative determination is made in step 228, steps 230 and 232
are skipped, and the process proceeds to step 234. In step 234, it is
determined whether or not there is a roller that is rotating among the
feed roller 32 and the rollers disposed along the transport path 30. If a
negative determination is made, the sheet length identification process
program ends. If a positive determination is made, the process proceeds
to step 236, where the rotation of rollers is stopped to stop the
transport operation, and then the sheet length identification process
program ends.

[0070] As described above in detail, in the image forming apparatus 10
according to the first exemplary embodiment, the length of a sheet of
recording paper P is estimated on the basis of individual detection
results generated by the respective first path sensor 91A and second path
sensor 91B disposed along the transport path 30, and an alarm signal is
output if the sheet length estimated on the basis of at least one of the
detection results is not a predetermined length. Accordingly, whether or
not the length of the transported sheet of recording paper P is the
predetermined length is identified with high accuracy, compared to a case
where the image forming apparatus 10 does not have the above-described
configuration.

Second Exemplary Embodiment

[0071] In the first exemplary embodiment, a description has been given of
the sheet length identification process in a case where a sheet of
recording paper P accommodated in the fourth accommodating unit 28 is
taken from the fourth accommodating unit 28 and is fed to the transport
path 30. In a second exemplary embodiment, a description will be given of
a case where any one of the first to fourth accommodating units 22, 24,
26, and 28 is specified, and a sheet of recording paper P is taken by the
feed roller 32 from the specified accommodating unit and is fed to the
transport path 30. The elements described above in the first exemplary
embodiment are denoted by the same reference numerals, and the
corresponding description will be omitted. Hereinafter, the points
different from the first exemplary embodiment will be described.

[0072] In the image forming apparatus 10 according to the second exemplary
embodiment, a sheet of recording paper P that is taken by the feed roller
32 from an accommodating unit and is fed to the transport path 30 may be
fed while being overlapped on the subsequent sheet of recording paper P
due to burrs or static electricity, for example. However, in the image
forming apparatus 10 according to the second exemplary embodiment, the
pairs of transport rollers 36 are provided for the respective first to
third accommodating units 22, 24, and 26. Thus, even if a sheet of
recording paper P is taken by the feed roller 32 from any one of the
first to third accommodating units 22, 24, and 26 and is fed to the
transport path 30 while being overlapped on the subsequent sheet of
recording paper P, the overlapped sheet may be separated by the pair of
transport rollers 36. Accordingly, a sheet of recording paper P that is
taken by the feed roller 32 from any one of the first to third
accommodating units 22, 24, and 26 and is fed to the transport path 30 is
less likely to be transported while being overlapped on the subsequent
sheet of recording paper P, compared to a sheet of recording paper P that
is taken by the feed roller 32 from the fourth accommodating unit 28 and
is fed to the transport path 30.

[0073] As described above, if there is a path along which a sheet is less
likely to be transported while being overlapped on another sheet, a
sufficient accuracy of identifying each sheet of recording paper P is
ensured for the sheets of recording paper P that are transported along
the path, even if an estimation result of a sheet length obtained on the
basis of the detection results generated by the first and second path
sensors 91A and 91B is not used, unlike in the above-described first
exemplary embodiment.

[0074] In the image forming apparatus 10 according to the second exemplary
embodiment, a sheet length identification process according to an
accommodating unit is performed, in which either of two estimation
methods is used. One of the two estimation methods is a method for
estimating the length of a sheet of recording paper P that is taken by
the feed roller 32 from any one of the first to third accommodating units
22, 24, and 26 and is fed to the transport path 30. The other is a method
for estimating the length of a sheet of recording paper P that is taken
by the feed roller 32 from the fourth accommodating unit 28 and is fed to
the transport path 30.

[0075] Hereinafter, the operation of the image forming apparatus 10 during
the execution of the sheet length identification process according to an
accommodating unit will be described with reference to FIGS. 7A and 7B.
FIGS. 7A and 7B illustrate a flowchart of an example of a process of a
sheet length identification process program according to an accommodating
unit executed by the CPU 150 when the rotation of the feed roller 32
corresponding to a specified accommodating unit is started in order to
take a sheet of recording paper P accommodated in the specified
accommodating unit and feed the sheet to the transport path 30. The sheet
length identification process program according to an accommodating unit
is stored in advance in a predetermined storage region of the ROM 152.
Here, in order to avoid complexity, a description will be given of a case
where any one of the first to fourth accommodating units 22, 24, 26, and
28 is already specified as an accommodating unit that accommodates sheets
of recording paper P to be transported. Also, a description will be given
of a case where the sheet length identification process program according
to an accommodating unit is executed in a state where the rotation of the
pair of positioning rollers 38 and the rollers disposed along the
transport path 30 is stopped.

[0076] Referring to FIGS. 7A and 7B, in step 400, it is determined whether
or not the accommodating unit that is currently specified is provided
with the pair of transport rollers 36 corresponding thereto. If a
negative determination is made, the process proceeds to step 402, where
the sheet length identification process program described above in the
first exemplary embodiment is executed, and the sheet length
identification process program according to an accommodating unit ends.

[0077] On the other hand, if a positive determination is made in step 400,
the process proceeds to step 404. In step 404, a step corresponding to
the above-described step 200 is performed. If a negative determination is
made in step 404, the process proceeds to step 406, a step corresponding
to the above-described step 238 is performed. If a positive determination
is made in step 404, the process proceeds to step 408. In step 408, a
step corresponding to the above-described step 202 is performed, and then
the process proceeds to step 410. In step 410, a step corresponding to
the above-described step 203 is performed, and then the process proceeds
to step 412. In step 412, a step corresponding to the above-described
step 204 is performed, and then the process proceeds to step 414. In step
414, a step corresponding to the above-described step 206 is performed,
and then the process proceeds to step 416. In step 416, a step
corresponding to the above-described step 208 is performed. If a negative
determination is made in step 416, the process proceeds to step 418,
where a step corresponding to the above-described step 240 is performed.

[0078] On the other hand, if a positive determination is made in step 416,
the process proceeds to step 420, where a step corresponding to the
above-described step 210 is performed, and then the process proceeds to
step 422. In step 422, a step corresponding to the above-described step
212 is performed, and then the process proceeds to step 424. In step 424,
it is determined whether or not the sheet length estimated in step 422 is
out of an assumed range. If a positive determination is made, the process
proceeds to step 426. In step 426, a step corresponding to the
above-described step 232 is performed, and then the process proceeds to
step 428.

[0079] On the other hand, if a negative determination is made in step 424,
the process proceeds to step 428. In step 428, a step corresponding to
the above-described step 234 is performed, and then the process proceeds
to step 430. In step 430, a step corresponding to the above-described
step 236 is performed, and then the sheet length identification process
program according to an accommodating unit ends.

[0080] As a result of executing the sheet length identification process
according to an accommodating unit, a sheet length is estimated with high
accuracy when a sheet of recording paper P is taken from any one of the
accommodating units and is fed to the transport path 30. Also, as for a
sheet of recording paper P that is taken from any one of the first to
third accommodating units 22, 24, and 26 and is fed to the transport path
30, the sheet length thereof is estimated on the basis of a detection
result generated by the first path sensor 91A, not on the basis of a
detection result generated by the second path sensor 91B. Thus, the load
of estimation is decreased compared to the case of estimating the sheet
length using the detection results generated by both the first and second
path sensors 91A and 91B. Also, the time for estimation is shortened.

[0081] In the image forming apparatus 10 according to the second exemplary
embodiment, when the assist function operation conditions are satisfied,
the assist function operates even though a sheet length specified as the
length of sheets of recording paper P to be transported is equal to or
larger than a predetermined sheet length and even though sheets of
recording paper P having a length shorter than the specified sheet length
are accommodated in the accommodating unit that is to accommodate sheets
of recording paper P of the specified sheet length. However, in a path
provided with the pair of transport rollers 36, plural sheets of
recording paper P that are fed by being overlapped are more likely to be
separated from each other while being transported, compared to a path not
provided with the pair of transport rollers 36. Thus, whether or not the
length of a sheet of recording paper P transported along the transport
path 30 is within an assumed range is identified with high accuracy on
the basis of a detection result generated by the first path sensor 91A,
not using a detection result generated by the second path sensor 91B.

[0082] In the above-described exemplary embodiments, a description has
been given of a case where a sheet of recording paper P accommodated in
an accommodating unit is transported. Alternatively, the exemplary
embodiments may be applied to a case where a sheet of recording paper P
held by the foldable manual sheet feeder 39 is transported. In the
example illustrated in FIG. 1, if sheets of recording paper P held by the
foldable manual sheet feeder 39 are taken by the feed roller 42 and are
fed to the auxiliary transport path 40 while being overlapped each other,
the sheets are likely to be separated from each other by the two pairs of
transport rollers 44 provided along the path leading to the pair of
positioning rollers 38. Thus, in the sheet length identification process
program according to an accommodating unit illustrated in FIGS. 7A and
7B, a positive determination is made in step 400, and steps 404 to 430
are performed.

[0083] In the above-described exemplary embodiments, a description has
been given of the case of estimating a sheet length on the basis of an
elapsed time from the execution of the transport start process.
Alternatively, a sheet length may be estimated on the basis of the time
period from when the first path sensor 91A shifts from the ON-state to
the OFF-state to when the first path sensor 91A shifts to the ON-state
again.

[0084] In the above-described exemplary embodiments, a description has
been given of a case where two sensors, that is, the first path sensor
91A and the second path sensor 91B, are provided along the transport path
30. Alternatively, three or more path sensors may be provided along the
transport path 30. In that case, sheet lengths are estimated on the basis
of detection results generated by the three sensors. If at least one of
the estimated sheet lengths is out of an assumed range, an alarm signal
is output. If all the estimated sheet lengths are within the assumed
range, an alarm signal is not output. Accordingly, whether or not a sheet
of recording paper P having a preset sheet length is being transported is
determined more accurately.

[0085] In the second exemplary embodiment, a description has been given of
a case where passing of a sheet of recording paper P taken from the
accommodating unit with which the pair of transport rollers 38 is
associated is detected by using the first path sensor 91A disposed on the
upstream side in the transport direction among the first path sensor 91A
and the second path sensor 91B. Alternatively, passing of the sheet may
be detected by using the second path sensor 91B disposed on the
downstream side in the transport direction. In this case, the sheet of
recording paper P passes the second transfer unit 37 and the fixing
device 100 before reaching the detection region of the second path sensor
91B. Thus, even if plural sheets of recording paper P are transported
there while being overlapped each other, the sheets are likely to be
separated from each other when passing the second transfer unit 37 and
the fixing device 100. Therefore, in the case of estimating a sheet
length by using a detection result generated by the second path sensor
91B, whether or not a sheet of recording paper P having a preset sheet
length is being transported is determined more accurately than in the
case of estimating a sheet length by using a detection result generated
by the first path sensor 91A, which is positioned on the upstream side in
the transport direction with respect to the second path sensor 91B. On
the other hand, in the case of estimating a sheet length by using a
detection result generated by the first path sensor 91A, whether or not a
sheet of recording paper P having a preset sheet length is being
transported is determined more quickly than in the case of estimating a
sheet length by using a detection result generated by the second path
sensor 91B, which is positioned on the downstream side in the transport
direction with respect to the first path sensor 91A.

[0086] In the above-described exemplary embodiments, a sheet of recording
paper P is transported. Alternatively, an overhead projector (OHP) sheet
or a color filter used for manufacturing a liquid crystal display may be
transported. That is, any type of sheet may be transported as long as an
image may be formed thereon.

[0087] In the above-described exemplary embodiments, various process
programs are stored in advance in the ROM 152. Alternatively, the various
process programs may be provided in the form of being stored in a
computer readable storage medium, such as a compact disc-read only memory
(CD-ROM), a digital versatile disc-read only memory (DVD-ROM), or a
universal serial bus (USB) memory. Alternatively, the various process
programs may be distributed via a wired or wireless communication medium.

[0088] The foregoing description of the exemplary embodiments of the
present invention has been provided for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise forms disclosed. Obviously, many modifications
and variations will be apparent to practitioners skilled in the art. The
embodiments were chosen and described in order to best explain the
principles of the invention and its practical applications, thereby
enabling others skilled in the art to understand the invention for
various embodiments and with the various modifications as are suited to
the particular use contemplated. It is intended that the scope of the
invention be defined by the following claims and their equivalents.